In many applications, fluid or gas carrying parts form critical components of a system. For example, in an automobile, fluid carrying parts couple the vehicle fuel supply to the engine intake and must be entirely free of leaks to prevent fuel spillage and a potentially dangerous condition. In a similar way, refrigerant carrying lines in a home air-conditioning system must be free of leaks so that refrigerant is unable to escape from the cooling system causing an air conditioning failure. The criticality of these parts mandates, in some instances, 100% testing of manufactured parts to verify the absence of leaks therein.
Pneumatic testing of a fluid carrying part or “unit under test” (UUT) typically involves filling the unit with gas at a specified target pressure, monitoring the pressure within the unit for a specified period, and then releasing the pressure within the unit and determining from the pressure profile experienced within the unit whether the unit is sufficiently leak free. Leaks are identified by an excessive reduction of pressure within the unit during the period that it is pressurized. To ensure that reductions in pressure will be accurately detected, after the unit is filled, it is isolated from the source of pressurized fluid and its pressure is monitored for a suitably lengthy period of time to detect a pressure drop caused by the smallest allowable leak.
While this method is an relatively straightforward to describe, its implementation in practical examples is fraught with the number of difficulties. Specifically, in a practical implementation, it is necessary that the entire pressure testing cycle including fill, test and exhaust be performed in the shortest possible time to allow for the greatest possible throughput of units being tested at the testing fixture. Throughput is particularly critical where all units must be tested on a fast-moving production line. If a long enough time is provided to stabilize the pressure within a unit prior to isolation of the unit from the pressurized source, transient thermodynamic effects will be eliminated, and the unit pressure will be well stabilized and any subsequent pressure change can be reliably attributed to leakage from the unit. However, in a product line environment where throughput is critical, there is a need to test the unit as rapidly as possible, without waiting a very long time for stabilization.
Unfortunately, rapid testing compromises the objective of obtaining a stabilized pressure within the unit prior to initiating a leakage test on the unit. This is due to a number of causes. First, the rapid influx of pressurized gas into the unit compresses the gas within the UUT, which causes the gas to release heat to the UUT, which then cools, causing a reduction of pressure within the unit after the unit has been filled. Cooling of gas within the unit will not alter the measured pressure within the unit so long as the unit remains connected to the pressurized gas source. However, once the unit is disconnected from the pressurized source, cooling of the gas within the unit will cause a pressure drop which can be easily confused with the pressure drop that is caused by leakage. A second difficulty arises from the fact that all units, including metal fluid carrying units, are subject to expansion and stretch when pressurized. Unit expansion may be caused by heat transfer from the pressurized fluid into the unit, and the unit may also deform elastically in response to applied pressure. Heat-related expansion and elastic deformation will be collectively referred to as “stretch” in the following. Unit stretch will also cause a reduction of pressure of the fluid filled unit, if the unit has been isolated from the pressurized source before such stretch effects have been fully dissipated.
FIG. 1 illustrates the features of the pressurization profile created by a pneumatic test fixture, that has been exaggerated in scale to show the combined effect of the various phenomena discussed above. Specifically, during an initial period 10, the pressure of the unit increases rapidly as the unit is brought into fluid communication of the pressurized source. The pressure of the interior of the unit will rapidly approach the pressure of the source, and remain at this pressure as long as the source remains in fluid communication with the interior of the unit. (It will be noted, however, that due to variation in the regulation of the pressure of the source, the actual unit pressure achieved will have variation that corresponds to the variation in the pressure source as seen at 10A, 10B and 10C; this possible source of variation is typically contained by the use of a pressure source that has sufficiently low regulation error that it brings each successive unit within an acceptably close proximity to a desired target starting pressure.)
Although the unit will be held at the source pressure so long as the source is connected, if the source is disconnected soon after filling, the unit's interior pressure will reduce due to the combined effects of unit stretch and cooling. These effects are illustrated in a stabilization region 12 of the pressure profile of FIG. 1. The extent of the pressure drop exhibited in region 12 of the curve is a function of the amount of stretch of the unit, and the amount of gas cooling that remains to be completed after the end of the initial pressurization period, in addition to any pressure drop caused by leakage. If the initial pressurization shown in portion 10 of the pressurization curve is performed for a longer period, unit stretch and fluid cooling will reach a greater level of completion during this period and the interior pressure of a non-leaking unit will be reduced to a lesser amount in region 12 after the fluid source is disconnected from the interior of the unit. Furthermore, it will be noted that any variation in regulation pressure will be reflected as an offset to the pressure curve in the stabilization region 12, as seen at 12a, 12b and 12c. 
At the completion of the settling behaviors that begin in region 12, the unit's pressure will stabilize, unless there is a leak in the unit. The divergence of pressure due to a leak will be apparent after the passage of time, represented in region 14 of FIG. 1, by the divergence of the pressure profile 14a of a non-leaking unit, from the pressure profile 14b of a leaking unit. Region 14 of FIG. 1 represents a far greater period of time then region 12, sufficiently long for a pressure leak to create a measurable divergence of the pressure profiles 14a dn 14b. It will be appreciated that a leak will also cause divergence in region 12, but over the time period represented in region 12 this divergence will be very small as compared to the effects of stretch and cooling.
To determine whether a part is leaking, therefore, after a sufficient time for stabilization in region 12, the test instrument may begin to monitor the pressure loss within the unit in region 14 to determine whether the pressure loss is within acceptable limits. Leak-free units will be characterized by a relatively stable pressure over time as seen at 14a, whereas leaking units will be characterized by a linear reduction in pressure of the interior of the unit that continues beyond stabilization, as shown at 14b. The testing period of region 14 must be sufficiently long that a reduction in pressure caused by the minimum size acceptable leak can be reliably identified during period 14, and furthermore, that the pressure reduction caused by a leak is sufficiently large to be reliably distinguished from variations in the stabilized unit pressure that are caused by normal variance in unit stretch, thermal effects, electrical transducer noise, operating temperature, and pressure regulation accuracy. That is, the test period of region 14 must be long enough for the pressure drop on curve 14b to be substantially greater than the lowest stable pressure that is likely to be seen on a leak-free unit, such as the relatively low stable pressure shown at 12c. 
After testing for pressure drops caused by leaks, by monitoring pressure for a suitable time period in region 14, the pressure within the unit is released causing a rapid reduction of pressure in region 16. Pressurizing gas is typically vented from the unit to a collection system, or alternatively vented to atmosphere.
As can be seen by examining the behaviors discussed above and illustrated in FIG. 1, it is necessary to choose time periods in regions 10 and 12 of the pressurization cycle that are sufficient to repeatably fill a unit and settle thermodynamic and stretch effects that may otherwise be mistaken for unit leakage. If, for example, the pressurization period 10 is made too short, the effect of cooling, heat transfer and stretch to the unit will cause too large of a reduction of pressure in region 12, causing the unit to be significantly outside of the target pressure for acceptance in region 14. However, it is not necessary to wait for all of the effects of heat transfer and stretch to dissipate, only long enough for those effects to be repeatable so they can be accurately subtracted from a leak rate calculation. Thus, it can be seen that region 14 in FIG. 1 begins prior to the completion of the stretch and cooling related pressure decays that begin in region 12. This is important as if an extremely long period is utilized in region 12 for pressurization or an extremely long period is utilized in region 14 for stabilization, throughput of the testing fixture will be dramatically reduced.
It is, however, necessary to choose a time period for testing in region 14 that is sufficient to accurately differentiate between a leaking unit having the minimum unacceptable leak, from variations that might be experienced during pressurization and stabilization of leak-free units. Too short of a test period might permit a leaking unit to pass through undetected, or force the use of a target pressure that rejects leak-free units that have a particularly low stabilized pressure.
In the past, the lengths of the pressurization period 10, settling period 12 and test period 14 have been chosen based on experience and trial and error, by test engineers observing the pressure profiles of the unit, and choosing an apparently effective duration of time for each of these phases of the test cycle. Unfortunately, this method typically produces a less than optimum timing for the pressurization cycle because portions of the cycle are often made longer than is necessary to achieve sufficient stability of the testing process for an effective test.
It is thus an object of the present invention to provide an automated methodology for selecting and controlling the periods applied to pneumatic cycle testing of a unit, and to provide a method of selecting those testing periods in a systematic way that produces more optimal and shorter cycle times for pneumatic testing.